Avalanche photodiode detection apparatus



45 5-61 AU 2 33 EX FIPBlOb 02 3,544,794 a i' Deck 1, 1979 w. T. LYNCH 3,544,794

AVALANCHE PHOTODIODE DETECTION APPARATUS (f -45 Filed Aug. 29, 1967 FIG. 3

EMS 0/\/ 0/005 /7 '/6 N p /4-I /2 FIG 2 lNVENTOR W 7. LVNC H ATTORNEY 3,544,794 Patented Dec. 1, 1970 United States Patent cc beams thus formed. Each diode then is separately biased 3,544,794 so that when one is avalanching the other is oil and vice AVALANCHE DETECTION versa. Thus, the application of the sampling theorem is avoided inasmuch as detection is afforded continuously. In the arrangement described above using two diodes each having a multiplication of M, the overall multiplication of the output signal will be, ideally, M+l/2. In other embodiments more than two diodes may be utilized to overcome departures from perfect square wave pulse forms in order to avoid significant periods in which de tection might not be effective.

In another aspect, some loss of information inherent Alternating current biasing to suppress microplasma i h F FmbPdimeHtS may be PY effects in avalanche photodiodes may be used even when P l g ophcal Swltchmg means for shlfimg h the signal frequency is greater than the bias frequency; lncominglightbeam from one photodlode to another in a condition which normally violates the sampling synchrqmsm f' blas frequencytheorem. The Sampling theorem requires that the signal The invention and its ob ects and features will be more be sampled, that is, measured, at least twice during dearly undefstoodfion} the followmg f' descljlp each time interval corresponding to the inverse of the h conluflctlon Wlth drawmg winch bandwidth For a wide bandwidth pulse detection System 00 FIG. 1 1s a schematic representation of a basic embodi- 1 t of the maximum fi ment of the invention and FIGS. 2 and 3 are graphs gii g is the equlva en showing the form of the bias voltage applied to the two diodes of the apparatus of FIG. 1.

Referring to FIG. 1, an incoming light beam 12 modu- 5 lated as by a train of pulses 11 impinges upon the halfsilvered mirror 13. At this point, the modulated beam is divided into two separate beams or channels 14 and Plased alternatfaly mto avalanche lireakdown thus i 15 inasmuch as part is transmitted and part reflected by mg Q l mlcroplasma Suppression but aliqcommuous the beam splitter 13. Each of the divided beams 14 and i i m the e mode and avoldmg the 15 exhibit the frequency of the incomingbeam 12 but Smchohs of the SamPhng theorem have only, ideally, one-half the intensity or amplitude of BACKGROUND OF THE INVENTION thenorigilrlial beam h d 0d d e c anne 1 en impinges on p oto i e 16 an Held of the mvemmn channel 15 on photodiode 17. Each of these photodiodes Recently improved detection of radiant energy has 16 and 17 is biased from a direct current potential source been achieved by the use of avalanche photodiodes. This 20 through resistor 22 and f alternating current improvement has particular significance with respect to potential sources 18 and 19 respectively. As set forth Optical communication Systems, WP Y those using more fully hereinafter sources 18 and 19 are out of beams of coherent light such as are produced by lasers. phase.

Description of the prior art In accordance with the patent of A. Goetzberger referred to hereinbefore, each diode is biased into avalanche Mhfe p y, avalanche photodlhde detechoh has breakdown cyclically so that the time during which the been improved by the use of alternating or cyclically v tage is above the microplasma breakdown voltage varying bias Voltages- AS disclosed 1n 3,454,435 is made small compared to the average turn-on time of gamed to A. Goetzhefgel' 011 July 1 a cyclically a microplasma. For example, in the case of a silicon Varying bias Voltage Suitably chosen Wlth lf to diode of N-i-P configuration having a bulk breakdown frequency and amplitude is effective in PP the of 25 volts in which there exists a small area microdeleterious effects of microplasmas. However, when such plasma, about 0-1 Square centimeters having a breakn arrangement is used with the frequency of the down voltage of 24 volts, a typical turn-on time for the coming signal to be detected greater than the bias fremicroplasma would be about 20 nano-seconds when the q y of the Photodeteclor, the ham samplme theorefn diode is operated at a peak voltage of 24.5 volts. If such of modulation theory indicates that some of signal will a diode is operated using an alternating current bias be lo t. T Sampling theorem q 'F the Signal h which swings from 23.5 volts to 24.5 volts, the average Sampled (measured) at least twice duhhg eachflme direct current will be substantially constant in value up terval corresponding to the inverse of the bandwidth. For

to about 10 Hz., but will be reduced by a factor of a wide bandwidth pulse detection system the bandwidth about 10 at about 108 Accordingly as taught in that is the equivalent of the maximum frequency. Accorddisclosure, the effect is to suppress microplasma current. ingly, this places a severe limitation on the use of this As is known in the art typically, such a Silicon diode advantageous mode of operation.

having an N+P configuration and a bulk breakdown SUMMARY OF THE INVENTION 0 of 25 volts would have a doping level in the P type sub- In accordance with this invention the above-described strate of 3.5 l0 atoms per cm. and a circular N+ region, 0.3 micron deep, having a doping level greater problem is solved by separating the incoming g than 10 atoms per cm. the entire circular region being which is in the form of a beam of radiant energy, into surrounded by a circular junction of higher breakdown a plurality of beams each of which is applied to a voltage, for example, a linear graded junction of less separate photodiode. Each photodiode then is separately than or equal to 10 atoms per cm. per cm. Such a biased using a frequency effective to suppress micrograded junction would have a breakdown greater than plasma effects and in a related manner so that at any or equal to 40 volts. one time at least one diode is in the avalanche detect- For silicon diodes having higher bulk breakdown values, ing condition. In 'a basic embodiment the incoming beam approximately equal to or greater than'SO volts, the turnmay be split into two beams using a half-silvered glass on times are longer and alternating current bias fremirror with a photodiode for detecting each of the two quencies of about 10 Hz. or less may be used to sup- William T. Lynch, Summit, NJ., assignor to Bell Telephone Laboratories, Incorporated, Berkeley Heights, 5 N .J., a corporation of New York Filed Aug. 29, 1967, Ser. No. 664,136 Int. Cl. H01j 39/12; H01] 15/06; H04b 9/00 US. Cl. 250-199 3 Claims ABSTRACT OF THE DISCLOSURE press the effect of microplasmas. This means also that for a given alternating current bias frequency, greater overvoltages above the microplasma breakdown value can be used for high breakdown voltage diodes than for low breakdown voltage diodes. Similar results have been obtained using germanium diodes.

In order that substantially continuous detection be afforded by the photodiodes 16 and 17, the alternating current bias voltage is timed using any of a variety of well-known techniques so that when one diode is on the other is off. Thus, as depicted in the graphs of FIGS. 2 and 3, the bias pulses as applied to diodes 16 and 17 are 180 degrees out of phase. Accordingly, the incoming beam 12 is subjected to continuous detection although at a somewhat, in this particular embodiment, reduced intensity. However, it will be apparent that the frequency of the incoming signal may be higher than the frequency of the bias applied to the diodes without any loss of information. For example, the incoming beam may have a carrier frequency of 5X10 Hz., the signal frequency may be 3x10 Hz. and the bias or sampling frequency may be 2 10 Hz. As these parameters are applied using a silicon diode as described above having a 25 volt bulk breakdown value the peak to peak voltage swing of the alternating current bias may be 1.5 volts and a suitable direct current bias is 24 volts.

In the particular embodiment of FIG. 1, if the multiplicationfactor of each diode alone in the avalanche condition is M, then the overall multiplication afforded for the recombined output signal will be M+1/2, in-

dicating the reduction occasioned by the beam splitting operation. However, some of this reduction may be regained in another arrangement in which means such as birefringent lens replaces the half-silvered mirror 13. The birefringent lens then is arranged so as to produce switching of the input signal beam 12 from one detecting diode to the other in synchronism with the frequency of the alternating bias voltage thus affording substantially continuous detection of essentially the full intensity beam.

Moreover, in another aspect of the invention more than two photodetecting avalanche diodes may be used by arranging optical means to divide or switch the incoming beam to impinge upon each diode. The important feature in accordance with the invention is to afford means whereby at least one diode is in the avalanche mode at all times. This arrangement involving a multiplicity of diodes is particularly advantageous where the alternating current bias voltage pulses depart from the ideal shape and render the apparatus susceptible to having periods between pulses when none of the diodes is avalanching. By utilizing a larger number of diodes, some overlap may be provided in the application of bias voltage to overcome the foregoing problem.

Although the invention has been described in terms of certain specific embodiments, it will be understood that other arrangements may be devised by those skilled in the art which still will fall within the scope and spirit of the invention.

What is claimed is:

1. Signal translating apparatus comprising optical means for dividing a signal beam of radiant energy into at least two channels, detection means for each channel comprising a semiconductor PN junction diode, and voltage means for biasing said diodes successively in reverse to the region of avalanche breakdown at a rate and ampliture so as to substantially inhibit continuous microplasma breakdown, characterized in that at least one of said diodes is in the avalanche breakdown condition at any one time, and means for combining the outut from said diodes into a common output signal.

2. Signal translating apparatus in accordance with claim 1 further characterized in that the frequency of the input signal is greater than the frequency of the voltage biasing the diodes.

3. Signal translating apparatus in accordance with claim 2 in which the optical means comprises a halfsilvered mirror for separating the incoming beam into two channels.

References Cited UNITED STATES PATENTS 3,300,644 1/1967 Zemel 307-311x 3,415,995 12/1968 Kerr 250-199 ROBERT L. GRIFFIN, Primary Examiner B. V. SAFOUREK, Assistant Examiner US. Cl. X.R. 

